100 at church finding hidden problem (5)
I believe the answer is:
catch
'hidden problem' is the definition.
('where's the catch?')
'100 at church' is the wordplay.
'100' becomes 'C' (Roman numeral).
'church' becomes 'ch' (common abbreviation).
'c'+'at'+'ch'='CATCH'
'finding' is the link.
(Other definitions for catch that I've seen before include "problem in small print?" , "Something trapped" , "Getting round" , "problem being concealed" , "Hidden drawback" .)